Instead of triple booting, just open Synaptic from Wubi's desktop (System -> Administration -> Synaptic Package Manager) and type your password to get in. Next, in the search box, type 'kubuntu-desktop' (with a dash in between kubuntu and desktop, not a space), click the checkbox by the kubuntu-desktop package and, finally, click Apply.
Now, whenever you want to switch DEs, in the login screen, under the Sessions menu (once you click your username) at the bottom of the screen, click 'KDE' and you'll be able to log into it. Similarly, to switch back to GNOME, just select GNOME from that same menu. This will save you from having to tri-boot.